iio: bmg160: Use i2c regmap instead of direct i2c access

This patch introduces regmap usage into the driver. This is done to
later easily support the SPI interface of this chip.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Reviewed-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>